1. Field of the Invention
This invention pertains to marine seismic receiving apparatus and in particular the use of multiple hydrostreamer cables for developing more definitive information from seismic reflections than obtainable from the use of a single hydrostreamer detection cable.
2. Description of the Prior Art
Current marine 2-D seismic surveys often utilize more than one hydrostreamer cable for reflection data gathering. A hydrostreamer cable, sometimes referred to as merely a streamer cable and sometimes also referred to as a hydrophone cable, is a long cable made up of sections having spaced therealong a plurality of hydrophones or hydrophone arrays for receiving reflected seismic reflections from the bottom of the sea and from subsurface interfaces. When more than one streamer is deployed by the same marine towing vessel, they are spaced apart and towed parallel to the surface of the water at the same depth or plane to give improved coverage. The usual depth of towing is about six meters underneath the surface of the sea.
The benefit from multiple-streamer surveys is that a swath-like coverage produces more data receiver points than can be provided by a single, linear cable and thereby allows the development of 3-D surveys. That is, hydrophones arranged in a plane allow improved seismic expression of the terrain subsurface or, in other words, allow the improved defining of dimensional characteristics of the terrain subsurface reflecting interfaces. In fact, the use of more than one streamer towed in a common plane has become sufficiently recognized as the means for the development of improved, definitive data, that the simultaneous towing in the same plane of three, four or even more cables is not uncommon.
It has been observed that swath-like land surveys have also proven to successfully provide more definition of subsurface characteristics than mere linear surveys, especially when directional geophones are utilized. A geophone is the term employed for a land survey sensor. It is well-known that such sensors or sensing instruments can be made and positioned for directional sensing. In more advanced and sophisticated land surveys, some geophones are aligned or positioned vertically, others are aligned horizontally and still others are aligned vectorily to allow for processing for azimuthal diffraction tracking.
In a marine environment, it is known that sea water will not support a shear wave front that is detectable by a horizontally aligned detector. Thus, at least one device, namely, a horizontally aligned detector, used in improving land survey information is not available. Marine surveys must rely, therefore, on high-quality compressional wave data.
As a result, sometimes even the best data developed using current marine detection procedures are confusing. For example, marine survey areas are often populated with fractured subsurface dipping beds that produce diffraction characteristics. Diffractions from out of a plane will interfere with energy returning from within such plane, thereby causing the mentioned confusion of data. Expensive 3-D surveys employing a dense coverage of the area with hydrophones are sometimes budgeted to permit the development of large amounts of data in a quantity sufficient to overcome the confusion and clarify the dimensional characteristics of the terrain subsurface covered by the survey.
Therefore, it is a feature of the present invention to provide for an improved marine seismic receiving apparatus capable of separating returning waveforms so as to provide dimensional clarification of subsurface terrain characteristics without an extensive number of receiver lines being required.
It is another feature of the present invention to provide for an improved marine seismic receiving apparatus including hydrostreamer cables towed at different depths.